1. Field of the Invention
This invention relates generally to the field of multiplexed digital data communications. More particularly, this invention relates to an access device for a digital network such as a digital service unit (DSU) for interconnecting a plurality of terminal devices running different applications and permitting them to share a single transmission channel in a multipoint multiport environment.
2. Background of the Invention
The term Digital Service Unit (DSU), as used herein, may in general also embrace combined Digital Service Unit/Customer Service Units (CSU), CSU's, or similar digital network access devices as will be appreciated by those skilled in the art. It may also embrace similar devices operating in digital data networks.
There are many business environments wherein multiple transmission lines are used to carry data to and from various terminal type devices. Typically, the total average data rate for these devices is less than the capacity of a single digital transmission line. Separate lines are often used because of protocol incompatibilities, separate applications being simultaneously run, gradual evolution of a communication network, connection to more than one Central computer, etc.
One typical example of such a system is that of a bank or other financial institution wherein at a single physical location there exists one or more terminals for use by tellers, terminals used by loan officers, accountants and the like for running other financial applications and automated teller machines (ATM). Another example is that of the retail industry wherein point of sale terminals (POST), credit verification terminals and accounting terminals may all use individual transmission facilities. Each of these may use their own dedicated analog or digital leased transmission lines which are not fully utilized. Conversion to a single digital multipoint multiport circuit may be more cost effective in many cases. Even if the transmission lines are fully utilized, conversion to a higher rate DDS service using a multipoint multiport circuit may result in substantial telecommunications cost savings.
When analog data modems are used, various techniques have been devised to facilitate sharing of transmission facilities and thus reduce telecommunications costs. For example, there are data modems which utilize frequency division multiplexing to divide a single transmission line into several channels. An example of such a scheme is shown in U.S. Pat. No. 4,335,464 to Armstrong et al. A time division approach for modems has also been proposed in European Patent Application number 88304437.2 published Nov. 23, 1988 under publication number 0292226. Another approach for modems has been described in U.S. Pat. application Ser. No. 07/355,521 assigned to the assignee of the present invention and incorporated herein by reference.
In order to achieve higher reliability in data communications at higher speed, many users are converting to all digital networks such as DDS networks. Multipoint (or Multidrop) circuits in DDS networks use multipoint junctions units (MJU's) or similar digital bridging devices to combine inbound data from each of the remote units.
An MJU (in a DDS network) allows two different modes of primary channel operation in the inbound direction. The mode is set in DDS-II by the control bit and in DDS-I by a bipolar violation sequence. In the first method (data mode), all remote stations transmitting to the Central station keep the primary channel in the data mode even if it has no primary channel data to transmit. In place of primary channel data, the Remote stations simply transmit all marks (all data bits set to logic ones). In this mode of operation, the MJU combines the data bits from different drops using the equivalent of a logic AND operation so that if any station transmits a zero, a zero will be passed to the Central. Otherwise, a logic one will be sent to the Central. This is the mode of operation which is used for the present invention.
In the second mode of operation (the control mode), the remote stations keep the data channel in the control mode by sending control mode idle (CMI) when there is not data to sent. In this mode, the remote station switches to data mode only when it has actual data to send. When the MJU receives a control code on any of the drops, it internally forces the data bytes from those drops to all marks (ones) prior to providing the logical AND bridging process. If all drops are inactive, the CMI sequence propagates to the Central. This second mode generally has the advantage that the CMI sequence can be used to distinguish between an active channel and an inactive channel thus providing DCD (Data Carrier Detect) control. This second mode of operation is conventionally the preferred mode of operation of a DDS network for multidrop operation in a DDS network.
In the case of DDS S/C, the MJU is also responsible for detecting secondary channel activity from a drop in order to bridge it with the primary channel data sent to the Central. In this case, however, the design of the MJU permits only one active secondary channel and ignores any other secondary channel activity from other drops.
For the preferred embodiment of the present invention, the network, and thus the access devices and MJU's, are used in the data mode rather than the control mode so that any remote station which is not transmitting data transmits all marks. Although using this mode does not provide the advantage of allowing simplified DCD control, it provides a convenient mechanism for permitting the alignment process of the present invention to be performed and multiport multidrop service to be provided.
The MJU's operating in the data mode basically perform a digital bridging function analogous to a logical AND operation on the primary data of the active channels in order to combine the data from the different points or drops in the circuit. This function may variously be referred to herein as an AND function, digital bridge function or MJU function synonymously and should not be strictly limited to the DDS definition of an MJU. The present invention is applicable to any digital network using similar digital bridging techniques. The MJU may be either embedded in the network, for example as part of a digital crossconnect system (DCS), or may be in the form of an MJU plug in card as will be appreciated by those skilled in the art.
Simple multi-point operation is contemplated by the DDS service providers and described in their various specifications. In simple multipoint operation, data from each remote does not need to be aligned in time since only one remote is polled by the Central (and therefore capable of transmitting) at any given time. However, time alignment of inbound data traffic may be required in some situations of multipoint multiport operation. For example, in multiport multipoint operation several remote sites may be polled simultaneously on different ports by their respective applications running at the Central site at any given time. Thus, the remote could transmit simultaneously during a portion of their inbound response to the poll if there is no time alignment of inbound frames. Since this would result in data errors, appropriate time alignment should be obtained.
More detailed technical information regarding the various Digital Data Systems may be obtained in the various technical specifications published by AT&T and other digital service providers for their digital data systems (e.g. AT&T Communications Technical Reference PUB 62120, 1984). Additional information is also available in U.S. Pat. No. 4,745,601 to Diaz et al, which is incorporated herein by reference.
The present invention provides a cost effective method and apparatus for accomplishing the multiport multidrop function in a digital network such as the DDS service provided by AT&T and provides for alignment of the inbound frames from remote DSU's. The present invention takes advantage of the characteristics of the digital bridging function to properly combine inbound multiport data. The present invention details a novel method to ensure the integrity of inbound TDM data from various drops of a DDS multidrop multiport network. This alignment method ensures that each bit transmitted from each remote arrives at the Central DSU properly mixed. Thus responses sent by different remote terminals connected to different remote DSU's can be kept in their appropriate TDM slots with respect to other neighboring TDM slots and not allowed to interfere with one another.